Starting with Lollipop, Google made the default email app its own Gmail rather than the AOSP email app that was in use for years. The functionality was largely the same, but some users experienced issues with the app syncing their Hotmail accounts after the update to Marshmallow. Thankfully, a Google employee in the Gmail help forum has stated that the team has found the root problem behind the syncing issue and will push the fixes into the next Gmail app update.